The above-identified United States Patent disclosed a novel programmable sequence controller employing a revolutionary programming method, herein called asynchronous drum programming, wherein a digital computer simulates a plurality of drums, each comprising a plurality of drum lines. Each drum is assigned to a particular output to be controlled and each drum line contains one or more logical statements providing exit conditions to further drum lines. This programming method allows ordinary maintenance people to program such programmable controllers for use in energy management systems.
All such programmable controllers hitherto comprised dedicated single unit systems providing for a limited number of inputs and outputs. This has limited the usefulness of these systems in large complexes such as energy management of large buildings and has increased the expense of their use due to the requirement to run input and output cables to the appropriate input sensors and controlled circuits.
Furthermore, such systems have been limited to specialized dedicated input ports, for example, so many input ports would be connectable to temperature sensors; so many connectable to switches to indicate whether they were open or closed; and so many connectable to a continuously variable voltage source.
Other limitations of prior art systems have included the high cost of buying a larger system than needed, due to lack of modularity; inability of the system to communicate the existence of commonly encountered situations requiring operator attention; the use of large, difficult to program, digital computers to control large systems; and, inability to diagnose the history of certain system failures.